In recent years there has been an intensive search for high strength aluminum which would satisfy the demands of advanced design in aircraft, automotive, naval and electrical industries. While high strength is a key characteristic of the materials sought, to meet the qualifications for certain advanced design applications the alloys must meet a combination of property requirements such as density, strength, ductility, toughness, fatigue and corrosion resistance, depending on the ultimate end use of the materials. The complexity of the problem goes far beyond the difficulties of developing materials with suitable combinations of properties not achieved before. Economics also plays a large role in the choice of materials. The ultimate product forms are often complex shapes, and the potential savings resulting from possible composition substitution is only a part of the picture. The new aluminum alloys would be particularly valuable if they could be shaped into desired forms using cost effective techniques such as forging while maintaining good characteristics and/or if they could be fabricated economically into the same complex shapes now used with other materials so as to eliminate the need for retooling for fabrication of weight saving structures.
The use of powder metallurgy routes to produce high strength aluminum has been proposed and has been the subject of considerable research. Powder metallurgy techniques generally offer a way to produce homogenous materials, to control chemical composition and to incorporate dispersion strengthening particles into the alloy. Also, difficult-to-handle alloying elements can at times be more easily introduced by powder metallurgy than ingot melt techniques. The preparation of dispersion strengthened powders having improved properties by a powder metallurgy technique known as mechanical alloying has been disclosed, e.g., in U.S. Pat. No. 3,591,362 (incorporated herein by reference). Mechanically alloyed materials are characterized by fine grain structure which is stabilized by uniformly distributed dispersoid particles such as oxides and/or carbides. U.S. Pat. Nos. 3,740,210, 3,816,080 (incorporated herein by reference) pertain particularly to the preparation of mechanically alloyed dispersion strengthened aluminum. Other aspects of mechanically alloyed aluminum-base alloys have been disclosed in U.S. Pat. Nos. 4,292,079, 4,297,136, 4,409,038, 4,532,106, 4,557,893 and 4,600,556.
For most uses a powder must be fabricated into a final product, e.g, by degassing, compaction, consolidation and shaping in one or more steps. To obtain complex parts the fabrication may take the form, e.g., of extruding, forging and machining. Usually, the less machining required to make a part the greater the economy in material use, labor and time. It will be appreciated that it is an advantage to be able to make a complex shape by forging rather than by a route which requires the shaping by manual labor on an individual basis.
It is academic that composition of an alloy often dictates the fabrication techniques that can be used to manufacture a particular product. In general, the target properties which must be attained in the type aluminum alloys of this invention before other properties will be considered are strength, density and ductility. One of the marked advantages of mechanically alloyed powders is that they can be made into materials having the same strength as materials made of similar compositions made by other routes, but with a lower level of dispersoid. This enables the production of alloys which can be fabricated more easily without resorting to age hardening additives. While the mechanical alloying route produces materials that are easier to fabricate than other aluminum alloys of comparable composition, the demands for strength and low density and the additives used to obtain higher strength and/or lower density usually decrease workability of the alloy system. (Workability takes into account at least ductility at the working temperature and the load necessary to form the material). The extent of the effect is generally related to the level of additive in the alloy. The additives not only affect the method by which the material can be fabricated, but also the fabrication techniques affect the properties of the materials.
In designing low density aluminum alloys, preferred additives are magnesium and lithium. These elements not only lower the density but also increase the strength of the aluminum. Lithium also increases the elastic modulus of aluminum. These highly useful effects are the basis for current interest in developing alloys of this type. However, efforts to develop high strength alloys of this type have been severely hampered by the propensity for these alloys to display relatively low tensile strength and low fracture toughness.
U.S. patent application Ser. No. 664,058, co-pending herewith discloses Al-Mg-Li alloys which are strengthened through mechanical alloying and then prepared in the forged condition. These alloys have useful properties and the processing route disclosed enables the possibility of using a wider range of conditions under which the materials can be forged and affords improved reproducibility of the forged parts. While the alloys disclosed have highly desirable properties, they have limitations. For example, lithium additions are far more effective in lowering the density of aluminum than any other element. Each percent of lithium added reduces density by about 3%. The maximum solubility of lithium in aluminum is about 4% at elevated temperatures, but drops to about 1.3% (wt. %) at room temperature. (Sanders & Starke, Aluminum-Lithium Alloys, AIME Proceedings, May 19-21, 1980.) In view of the benefits of lithium addition, it is desirable to add as much lithium as possible. However, if lithium is increased above the solubility limit the alloys become age hardenable and susceptible to embrittlement in service. In the alloy system of the present invention, which incorporates silicon in Al-Mg-Li system, the silicon decreases the likelihood of damaging embrittlement. Thus, it is possible to gain the advantages of incorporating higher amounts of lithium while decreasing the embrittling effect, thereby obtaining alloys of decreased density and good ductility.